51�Թ�

The first challenge was to dock safely and successfully at the ISS, which they managed to do within several minutes after Mission Control on the ground helped them restart the craft's thrusters.

This included issues with some of Starliner's thrusters, which would be needed to slow the spacecraft for re-entry into the Earth's atmosphere, and leaks of helium gas in the propulsion system.

"It doesn't sink," says Mr Heald, explaining how it uses four horizontal and vertical thrusters to move in any direction.

However, technical issues including helium leaks and thruster malfunctions meant that the Starliner was unsafe for their return.

It found that crew members had believed there were problems with the ship's thruster when they could not change the vessel's direction.